The present invention relates to a method and apparatus for determining the position of the poles of a synchronous linear stator motor.
A preferred application and embodiment of the invention is the drive of a magnetic suspension vehicle in which the vehicle represents the rotor containing the magnet poles of a synchronous motor while a rail installed along a track is designed as a linear stator and contains the stator windings installed in slots and fed by an inverter. If the flux entering the stator at a given point is designated with .phi..sub.d and a point shifted by a quarter period according to the periodic pole structure of the rotor with .phi..sub.q, the flux generated by the stator can be described as a vector with the two orthogonal, rotor-related components .phi..sub.d and .phi..sub.q, while the axes d and q, which are shifted magnetically relative to each other by 90.degree., are correlated to the corresponding points at the stator. Since the geometry of the stator winding is tuned to the pole pitch of the rotor, the relative position of the vehicle within a period of the spatially distributed stator field can be determined by a flux measurement in the rotor (i.e., by a stray flux measurement if the spatial distance from the rotor magnets is large enough).
The vehicle can now be accelerated by means of the magnetic flux, for the generation of which a current is impressed on the stator windings which is given by a certain current distribution corresponding to the accelerating flux. The current to be impressed can likewise be described by two current components i.sub.d and i.sub.q which are described as orthogonal components of a current vector in a coordinate system, referred to the position of the rotor. For presetting the current distribution corresponding to the desired acceleration moment, the desired values for these current components referred to the rotor position can therefore be preset, for instance, by means of a speed control. For controlling the inverters feeding the stator, it is then only still necessary to convert these desired current values referred to the rotor by means of a pole position signal indicating the position of the rotor, into stator-related reference values.
For forming this pole position signal, a slot counter which is arranged in the vehicle and delivers a counting pulse at each passage of a stator slot is shown in FIG. 1 of German Patent Application No. 31 48 007. A pulse train with a speed proportional frequency is generated, from which the pole position signal corresponding to the distance travelled (and therefore to the pole position) is generated by summing. Since, however, some slots may be missing in the stator at switches or other design-related points, the summed counting pulses do not exactly correspond to the pole position. It is therefore provided to measure the angle determining the rotor related stator current density (ampere turns per cm) by means of a measuring device for the stator stray flux and to divide the corresponding angle signal by a stator current signal which is formed at the inverter output by the actual values of the stator currents and indicates the stator-related stator current angle. Thereby, a position signal is formed which is independent of the amplitude of the stator current and by which the counter for the counting pulses is reset with the respective period of this position signal.
If the vehicle enters a line section in which the stator current feed is omitted because of a fault, then the stator stray flux measuring device measures only the residual magnetization of the section and possibly the stray field of the rotor poles, i.e., an irregular measuring signal, while actual stator current values are not available at all. The correction signal required for resetting the counting-pulse counter therefore cannot be formed; the counter rather resets itself periodically.
This known device, however, has the disadvantage that the angle signal for the rotor-related stator current density formed by the stator stray flux measuring device can in many operating states be determined only inaccurately but affects the determination of the pole position signal very significantly via the counter resetting command.